Thermosetting resins are widely used in the formation of fiber-reinforced structural materials due to their ability to withstand high temperatures. These materials provide the high strength and high temperature capability needed in such applications as military aircraft.
Thermosetting resins which have been used in this context include epoxy resins and bismaleimide resins. The latter are particularly beneficial due to their high glass transition temperature, their high capability of withstanding hot wet environments, and their low smoke and toxicant emission.
The beneficial properties of thermosetting resins are unfortunately offset by a brittleness or low toughness resulting in low damage tolerance. Toughness is extremely important in high-performance structures, since high loads must be borne both within the plane of the composite and in directions normal to the plane of the composite. Among the solutions offered for increasing toughness are the use of thermoplastics in combination with the thermosetting resins. This usually occurs, however, at the expense of processability, the thermoplastic additive causing an increase in the viscosity and the softening point of the material.
Patent application Ser. No. 07/312,016, filed Feb. 16, 1989, discloses resins made from a continuous phase of thermosetting resin containing a dispersed phase of thermoplastic material, the thermoplastic material being one which is solid at ambient temperatures but which dissolves in the thermosetting resin at a temperature below the gelation temperature of the thermosetting resin. The term "gelation temperature" as used herein refers to the temperature at which gelation occurs in the normal heat processing of the resin to achieve a thermal cure, as distinguished from any gelation which might occur very slowly at lower temperatures or during storage.
According to the present invention, the thermoplastic particles in the procedure of patent application Ser. No. 07/312,016 are replaced by particles which are a single-phase blend of at least two thermoplastics from at least two distinct classes. It has indeed been discovered that the resin which results when these particles are dispersed in a matrix of thermosetting polymer and then cured has unique properties in terms of toughness, processability, utility at high temperatures and other performance and use characteristics.
The blend is a mutually compatible combination of thermoplastics, at least one from each of the following two classes of thermoplastics:
(i) a thermoplastic polymer with a glass transition temperature (T.sub.g) of at least about 250.degree. C., and
(ii) a thermoplastic polymer with a fracture toughness of at least about 2.5 ksi .sqroot.in, and a glass transition temperature of less than about 240.degree. C. These values of T.sub.g represent the polymer species when molded individually, although this is not the form which they assume either in the particles or in the final product.
As used herein, the term "compatible" designates species which are miscible such that when combined, these species form a single homogeneous phase. The term "blend" designates a combination of the species, such that any single particle of the blend contains both species intimately mixed together in a single homogeneous solid phase, in contrast to mixtures of particles where each particle contains only one such species. The blends herein are thus analogous to alloys in the metallurgical context.
In contrast, the combination formed by first combining the thermoplastic blend particles with the fluid thermosetting resin precursor and then curing the combination at elevated temperature is not homogeneous. The cured combination instead exhibits a two-phase morphology, one such phase rich in thermoplastic material and the other rich in thermoset material. As will be explained below, the phases are often inverted upon cure, the dispersed phase being the phase rich in thermosetting material and the continuous phase being the phase rich in thermoplastic material.
The term "composite" is used herein to denote the combination of the two-phase thermosetting/thermoplastic resin and reinforcing material coated or impregnated with the resin.
Critical to this invention is the inclusion of at least one thermoplastic species from each of the two classes designated above. The present invention further extends, however, to the inclusion of additional thermoplastics compatible with these two, the additional thermoplastics either falling within one of the two classes or falling outside both. In these cases, the thermoplastic blend is a combination of three or more species. In preferred embodiments of the invention, however, the combination is limited to one from each of the two classes designated above.
It has further been discovered that by varying the weight ratio of the two thermoplastic components in the particles, one can control the properties of the final thermosetting/thermoplastic resin which is formed when the particles are combined with the thermosetting material precursor and the entire combination is cured. This is a further unexpected feature, not predictable from the known qualities of these resins, since it permits one to tailor the properties of the final resin to specific needs and performance requirements by adjustment of the relative amounts of the thermoplastic components alone. It is particularly surprising and nonobvious that the thermoplastics, when combined in the blend, yield a composite which displays a balance of properties far superior to those of the individual components standing alone.
Other features, advantages and further aspects of the invention will be apparent from the description which follows. The overall resin finds utility both as an adhesive and as the resin component in structural composites.